Digital subscriber line (DSL) technologies can provide large bandwidth for digital communications over existing subscriber lines. When transmitting data over the subscriber lines, some DSL technologies, such as Asymmetric DSL (ADSL) or Very high rate DSL (VDSL), use a Discrete Multi-Tone (DMT) line code that allocates a plurality of bits for each sub-carrier or tone in each symbol. The DMT may be adjusted to various channel conditions that may occur at each end of a subscriber line. In such technologies, typically three power management states may be defined, such as in the Telecommunication Standardization Sector (ITU-T) G.992.3/5 and G.993.2 specifications for ADSL2 and VDSL2, respectively, all of which are incorporated herein by reference. The three power management states comprise a mode 0 (L0) state for normal operation mode, a mode 2 (L2) state for idle mode, and a mode 3 (L3) state for off mode.
Statistically, the DSL link is idle most of time. L2 state is designed in G.992.3/5 such that the transceivers can take advantage of that factor to reduce the power consumption by reducing the transmitted signal level. To reduce the power consumption, the DSL transceiver should be maintained more frequently in the L2 state rater than the L0 state at a relatively low-speed connection, which reduces the transmission power spectrum density (PSD) at the corresponding line. Reducing the transmission PSD reduces the power consumption of the line driver, which comprises a relatively large portion of the total power consumption in the system. Additionally, reducing the transmission PSD reduces the crosstalk interference between transmitted signals over adjacent twisted-pair phone lines in a same or nearby bundle of lines. When the DSL devices on a subscriber line detect a reduction in crosstalk interference, the DSL devices may adjust their operations, for instance by increasing their data rate or reducing their transmission PSD. However, when the line in L2 state returns to L0 state, its transmission PSD is increased to the normal level, thus the crosstalk to the neighboring lines increases. Since those lines may have adjusted their operation based on the reduced crosstalk level, they may not operate properly under the increased crosstalk level, causing network instability. Therefore, a better low power mode is needed which does not cause network instability.